FR2800379A1 - PROCESS OF GAS-PHASE COPOLYMERIZATION OF AT LEAST TWO ALPHA-OLEFINS HAVING 2 TO 12 CARBON ATOMS - Google Patents

Abstract

The present invention relates to a process for controlling the density of copolymer produced by a process for the copolymerization of alpha-olefins in a polymerization reactor characterized in that the control is obtained by maintaining the ratio of the comonomer introduction flow rates (s ) constant monomer.

Description

The present invention relates to a method for controlling the density.

  of copolymer produced by a process of copolymerization of alpha-olefins in a polymerization reactor characterized in that control is obtained by maintaining

  the ratio of the flow rates of introduction of comonomer (s) to the constant monomer.

  0 It is well known to copolymerize olefins, for example ethylene, propylene, butene, hexene, octene, continuously, for example in the gas phase in a fluidized bed reactor or mechanically stirred, in presence of a polymerization catalyst, for example a catalyst of the Ziegler-Natta type, a

  metallocene, a chromium catalyst, an iron or cobalt catalyst.

  Many methods have already been described in the literature with regard to controlling the polymerization reaction. By proceeding to the copolymerization of alpha-olefins in a polymerization reactor in the presence of a metallocene catalyst, the Applicant has not succeeded, using known methods, in effectively controlling the density of the copolymer produced,

  as evidenced by the comparative example described below.

  Unexpectedly, the Applicant has found that the density of a copolymer produced by gas phase polymerization in the presence of a metallocene catalyst can be effectively controlled by maintaining the flow rate ratio

  introduction of comonomer to constant monomer.

  -2- The present invention therefore relates to a process for controlling the density of copolymer produced by a process for the copolymerization of alphaolefins having 2 to 12 carbon atoms in a gas phase polymerization reactor in the presence of a type catalyst. metallocene characterized in that control is obtained by maintaining the ratio of the flow rates of introduction of comonomer (s) to

constant monomer.

  We can represent this characteristic by the formula (qCi / qM) = K in which qCi is the rate of introduction of the comonomer i into the reactor, qM

  is the rate of introduction of the monomer into the reactor and K is therefore a constant.

  In its interpretation within the meaning of the present invention, the term “monomer M” means the olefin whose molar concentration in the polymer is the highest; by deduction, the comonomer Ci is any olefin whose molar concentration in the polymer is lower than that of the monomer M. According to the present invention, the term constant means a ratio which would not vary by more than 10%. preferably not more than 5%., more preferably

  not more than 2%, under normal operating conditions.

  The main advantage of the process of the present invention is that it makes it possible not only to obtain copolymers having a property of constant density over time but also to control in a simpler and

  effective copolymerization than in the past.

  The composition of the reaction gas mixture which passes through the copolymerization reactor, preferably the fluidized bed reactor, therefore contains at least two olefins which may, for example, have from 2 to 12 carbon atoms such as ethylene, propylene, butene. 1, hexene-1, methyl-4 pentene-1, octene-1. Preferably, the monomer is ethylene or propylene, and the comonomer is propylene, butene I, hexene-1, 4-methyl pentene-1 or octene-1. More preferably, the monomer is ethylene and the comonomer is butene-1,

  hexene-1 or 4-methyl-pentene-1, preferably hexene-1.

  The reaction gas mixture can also contain an inert gas such as nitrogen and / or a saturated hydrocarbon such as ethane, propane, butane, pentane,

hexane, and hydrogen.

  The polymerization is advantageously carried out continuously in a fluidized bed reactor according to techniques known per se and in apparatuses such as those described in French Patent Nos. 2,207,145 and No. 2,335,526 or European Patent No. EP-0 855 411. The reaction gas mixture containing the alpha-olefins to be polymerized is generally cooled by means of at least one heat exchanger placed outside the reactor before being recycled using a recycling line. The process of the invention is particularly suitable for very large industrial reactors; according to one embodiment of the present invention, the reactor used makes it possible to produce quantities of copolymers greater than 300 kg / h, preferably greater than 10,000 kg / h. According to a preferred embodiment of the present invention, the polymerization reactor is also fed by the catalyst with a constant flow of catalyst, which also facilitates the control of the activity of the polymerization reaction. In fact, such conditions unexpectedly lead to the production of copolymer having physicochemical characteristics.

  constant, which is crucial for an industrial process.

  Quite surprisingly, the Applicant has found that the control process which it had developed for metallocene catalysis in the gas phase could also be extended to other polymerization catalysts and to other types of processes. polymerization (for example in suspension) provided that said copolymerization process meets certain essential conditions. Indeed, if one finds oneself in situations of copolymerization for which the ratio of the molar concentrations of the comonomers to the monomer in the copolymer is greater than the ratio of the partial pressures of the comonomers to the monomer, then the density control according to the present invention can

advantageously be used.

  This condition will be represented by the formula ([Ci] / [M])> (pCi / pM) in which [Ci] is the molar concentration of comonomer i in the polymer, [M] is the molar concentration of monomer M in the polymer, pCi is the partial pressure of comonomer i and pM is the partial pressure of monomer M. The present invention therefore also relates to a control method

  the density of copolymer produced by an alpha- copolymerization process

  olefins having from 2 to 12 carbon atoms in a polymerization reactor in the presence of a polymerization catalyst characterized in that the ratio of the molar concentrations of the comonomers to the monomer in the produced copolymer is greater than the ratio of the partial pressures of the comonomers to the monomer and in that the control is obtained by maintaining the ratio of the flow rates of introduction of comonomer (s) to the constant monomer, namely in that ([Ci] / [M])> (pCi / pM) and (qCi / qM) = K. 25. According to a preferred embodiment of the present invention, the polymerization reaction is carried out in the gas phase, preferably in a fluidized bed reactor. -5- The monomers and comonomer (s) are preferably chosen from

  olefins having 2 to 12 carbon atoms such as ethylene, propylene. butene-

  1, hexene-1, methyl-4 pentene-1, octene-1.

  According to the preferred process of the present invention, in the gas phase polymerization reactor, the total pressure of the reaction gas mixture is usually between 0.5 and 5 MPa, preferably between 1.5 and 2.5 MPa; it can vary freely, preferably with maximum variations of less than 0.3 MPa and, in most cases of the order of 0.1 MPa. In fact, it is obvious that for safety reasons the pressure of the reaction gas mixture will not be allowed to exceed a predetermined maximum pressure which generally depends on the reactor used. Thus, it will be possible to reduce the flow rates of (co) monomers (while preferably keeping the flow rate ratio constant in accordance with the present invention) and / or to increase the catalyst injection flow rate in the case o the pressure of the reaction gas mixture reaches the

maximum pressure.

  It is also obvious that the pressure of the reaction gas mixture must be maintained above a predetermined minimum pressure in order to allow minimal and sufficient elimination of the heat of polymerization. In a fluidized bed reactor, this minimum pressure must also allow good fluidization of the polymer particles forming the bed. An inert gas having a good heat exchange capacity can advantageously be used in order to reach this pressure

  minimal. According to the process of the present invention, the partial pressure of alpha-

  olefins can also vary freely.

  The copolymerization can therefore be carried out for example in the presence of a catalyst of the Ziegler-Natta type comprising at least one transition metal, associated with a cocatalyst comprising an organometallic compound, for example an organoaluminum compound. The catalyst essentially comprises an atom of a transition metal chosen from the metals of groups IV to VI of the Periodic Classification of the elements, such as titanium, vanadium, chromium, zirconium or hafnium, possibly a magnesium atom and a halogen atom. The catalyst can be supported on a porous refractory oxide such as silica or alumina, or be combined with a solid magnesium compound, such as chloride, oxide, hydroxychloride or a magnesium alcoholate. By way of example, mention may be made of the catalysts described in patents US4260709, EP0598094, EP0099774 and EP0175532. The present invention is also particularly suitable for Ziegler's catalysts supported on silica, for example those described in patents WO9309147, W'09513873, WO9534380 and

W09905187.

  It is also possible to use a catalyst complexed with a metal selected from those of group VIII of the Periodic Table of the Elements. such as, for example, nickel, iron or cobalt, examples of those described in patent application WO98 / 27124 or WO98 / 2638 will be cited as examples. It is also possible to use catalysts based on platinum or palladium as the transition metal; of

  complexes of this type are described for example in patent WO 9623010.

  According to the preferred embodiment of the present invention, the catalyst for

  copolymerization is a metallocene type catalyst.

  By way of example, mention may be made of those corresponding to the formula [L] mM [A] n in which L is a bulky ligand; A is a leaving group, M is a transition metal and m and n are such that the total valence of the ligand corresponds to the

valence of the transition metal.

  Ligands L and A can be bridged. L is generally a cyclopentadienyl ligand. Examples of such metallocene catalysts are described in U.S. Patent Nos. 4,530,914, 5,124,418, 4,808, 561, 4,897,455, 5,278,264, 5,278,119,, 304,614, and EP-A-0 129 368, EP-A-0 591 756, EP-A-0 520 732. EP-A-0 420 436,

  WO 91/04257 WO 92/00333, WO 93/08221, WO 93/08199.

  "7- Metallocene-based catalytic systems can also be advantageously used as described in U.S. Nos. 4,871,705, 4,937,299, 5,324,800,

  , 017.714 4,5,120,867,4,665,208, 4,952,540, 5,091,352, 5,206,199, 5,204, 419,

  4,874,734, 4,924,018, 4,908,463, 4,968,827, 5,308,815, 5,329,032, 5,248, 801,

  5,235,081, 5,157,137, 5,103,031 and EP-A-0 561 476, EP-BI-0 279 586, EP-A0 594-

218 and WO 94/10180.

  Mention may also be made of patents WO 92/00333, WO 94/07928, WO 91/04257, WO 94/03506, U.S. Nos. 5,057,475, 5,096,867. 5,055,438, 5,198,401, 5,227, 440, 264,405, EP-A-0 420 436, U.S. Nos. 5,064,802, 5,149,819, 5,243,001, 5,239,022,

  5,276,208, 5,296,434, 5,321,106, 5,329,031, 5,304,614, WO 93/08221, WO

  08/13/199 and EP-A-0 578 838. The preferred transition metal compounds of

  catalysts are those of Group 4, in particular, zirconium, titanium and hafnium.

  The metallocene catalyst used in the present invention can also be represented by the general formula (Cp) m MRnR'p in which Cp is a ring of cyclopentadienyl type, M is a group 4, 5 or 6 transition metal; R and R 'can be chosen from halogens and hydrocarbyl or hydrocarboxyl groups m = 1-3, n = 0-3, p = 0-3. and the sum m + n + p equals the oxidation state of M; of

preferably m = 2, n = I and p = 1.

  The metallocene catalyst used in the present invention can also be represented by the general formula (C5 R'm) p R "s (C5 R'm) Me Q3-px, or R" s (C5 R'm) 2 Me Q 'in which Me is a group 4, 5 or 6 transition metal, at least one C5 R'm is a substituted cyclopentadienyl, each R', which may be the same or different, is hydrogen, an alkyl radical, alkenyl, aryl, alkylaryl or arylalkyl having from I to 20 carbon atoms or two carbon atoms linked together to form a part of a substituted or unsubstituted ring having from 4 to 20 carbon atoms, R "is a radical containing a or several or a combination of carbon, germanium, silicon, phosphorus or nitrogen atom which bridges two rings (C5 R'm), or which bridges a ring (C5 R'm) to M. when p = 0, x = l, otherwise "x" is always -8- equal to 0, each Q which may be the same or different an alkyl, alkenyl, aryl, alkylaryl or arylalkyl radical having from I to 20 a carbon tomes, a halogen or an alkoxides, Q 'is an alkylidene radical having from I to 20 carbon atoms, s is 0 or 1 and when s is 0, m is 5 and p is 0, 1 or 2 and when s is 1, m is 4 and p is 1. Metallocene catalysts are generally used with an activator or cocatalyst. Examples include alumoxane and / or neutral or ionic ionizing activators, or compounds such as tri (n-butyl) ammonium pentafluorophenyl tetraborate or the boric metalloid precursor of 1o trisperfluorophenyl, which ionizes the neutral metallocene compound. Compounds of this type are described in EP-A-0 570 982, EP-A-0 520 732, EPA-0 495 375, EP-A-0 426 637, EP-A-500 944, EP-A-0 277 003 and EP-A-0 277 004, and US Nos. 5,153,157,, 198,401, 5,066,741, 5,206,197 and 5,241,025, WO 94/07928. Combinations of catalysts can also be used, for example those described in U.S. No. 5,281,679, U.S. Nos. 4,701,432, 5,124,418, 5,077,255 and

, 183.867.

  Other examples of metallocene catalysts are described in U.S. Patents No. 317,036, EP-A-0 593 083, U.S. Nos. 4,937,217, 4,912,075, 4,935,397,

  4,937,301, 4,914,253, 5,008,228, 5,086,025, 5,147,949, 4,808,561, 4,897, 455,

  4,701,432, 5,238,892, 5,240,894, 5,332,706, WO 95/10542, WO 95/07939,

WO 94/26793 and WO 95/12622.

  Preferably, the metallocene comprises A) an inert support, B) a metal complex from Group 4-10 corresponding to the formula: Cp -MX zo M is a metal from one of groups 4 to 10 of the Periodic Table of Elements, Cp is an anionic ligand group, -9- Z is a divalent part linked to Cp and linked to M, comprising boron or an element from Group 14 of the Periodic Table of the Elements, and also comprising nitrogen, sulfur phosphorus or oxygen; X is a neutral conjugated diene ligand group having up to 60 atoms, or a dianionic derivative, and C) an ionic cocatalyst capable of converting the metal complex into a catalyst

active polymerization.

  Examples of cocatalysts are described in US 5,132,380, 5,153,157,, 064,802, 5,321,106, 5,721,185, and 5,350,723.

  Mention may also be made of the complexes described in WO96 / 28480 and WO 98/27119.

  The catalyst can be used in the form of prepolymer prepared in advance during a prepolymerization step from the catalysts described above. The prepolymerization can be carried out by any process, for example, a prepolymerization in a liquid hydrocarbon or in the gas phase

  according to a discontinuous, semi-continuous or continuous process.

  The catalyst or prepolymer can be introduced into the reactor of a

continuously or discontinuously.

  Those skilled in the art have at their disposal various techniques making it possible to determine the concentration of comonomer in the final polymer. Examples include nuclear magnetic resonance and

infrared spectroscopy.

  The method used in the context of the examples described below is that of

infrared spectroscopy.

  The comonomer content measurements were obtained by measuring the intensity of the infrared absorption bands obtained by transmission from films.

  thickness tablets ranging from 200 to 250 gm.

  The calibration was carried out using polymers characterized by NMR spectroscopy. After correction of the baseline, the comonomer contents were derived from the ratios of the different absorption bands as follows: butene-1 A772 / A4320 hexene- 1 A1377 / AI1368 4-methyl pentene-1 A920 / A4320 Ay corresponding to the absorbance observed for a wave number equal to y cm-l For the measurement of hexene-1, the absorbance at 1377cm'1 includes the contributions of all the methyl groups including those located on connections n -butyle and on the chain ends. A correction was therefore applied to the raw data in order to take account of the n-butyl connections, and therefore of the quantity of hexene-1 in the polymer. This correction is based on the value of the number average molecular weight, Mn, taking into account that the polymer contains 2 terminal methyl groups.

  The following examples illustrate the present invention.

-the-

Example 1

  The operation is carried out in a conventional fluidized bed reactor consisting of a vertical cylinder 5 m in diameter and 18.5 m in height. This reactor was previously purified so as to reduce the content of poisons in the reaction gas mixture used, according to the method described in Example I of the European patent application.

EP-A-180 420.

  The reactor initially contains a fluidized bed with a height of 10 m consisting of a polymer originating from a previous reaction and having a density of 0.92, a 0o index of fluidity IF 2.16 measured under 2.16 kg and at a 190 C temperature of 2.4 g per 10 minutes, a molecular weight distribution of 3.7, a titanium content of 5

  ppm, and a butene-1 content of 9%.

  Initially, the reaction gas mixture passing through the fluidized bed contains by volume 60% nitrogen, 60% ethylene, 0.27% hexene-1 and 0.15% hydrogen. The initial total pressure of this mixture is 2 MPa and the fluidization speed is

  52 cm / s. The temperature of the polymerization reaction is then 75 C.

  A catalytic system is used as indicated in Example 1 of the application for

  patent application number GB 9910370.7.

  hours after starting the reaction, the total pressure is 2 MPa, the fluidization speed is always 52 cm / s and the height of the fluidized bed is 12 m. Furthermore, the reaction temperature is 75 ° C. and the content of titanium in the polymer

manufactured is 3 ppm.

  At this time, the ethylene introduction rate is 0.5 T / h; the system for regulating the flow rates of introduction of ethylene and hexene-1 is controlled so that the ratio of these flow rates (qC6 / qC2) is constant; in this case this report

is 0.1.

  -12- Then, every hour, the ethylene flow rate is increased by 500 kg / h; the flow rate of hexene-1 is increased simultaneously in order to keep the ratio of the flow rates of introduction to the value 0.1. At the same time, the catalyst flow rate is increased by g / h. After 30 hours, the final total pressure is 2.4 MPa, the reaction temperature is 75 C, the fluidization speed is 55 cm / s and the height of the fluidized bed is 19 m. A polymer having the characteristics of the targeted polymer powder is withdrawn at a rate of 16.5 t / hour. This production rate is then

kept constant.

  One can observe a production of withdrawn polymer having a remarkable consistency of quality, in particular the density as shown in Figure 1. In addition, no formation of agglomerates or fine particles disturbed the operations. The percentage of hexene-1 by weight in the copolymer is 8%, the density of the copolymer is 0.916 and the melt index IF2.16 measured under 2.16 kg

  and at a temperature of 190 C is 1.3 g per 10 minutes.

Comparative example.

  The operation is carried out in a fluidized bed reactor identical to that of the previous example and previously purified in a similar manner and under identical conditions, namely: - initial bed height 10 m - polymer bed originating from a previous reaction and having a density of 0.92, a melt flow index IF2.16 measured under 2.16 kg and at a temperature of 190 C of 2.4 g per 10 minutes, a molecular weight distribution of 3.7, a titanium content

  of 5 ppm, and a butene-1 content of 9%.

  - identical catalyst - identical reaction gas mixture - initial total pressure of 2 MPa - fluidization speed of 52 cm / s

  - polymerization temperature of 75 C.

  -13- hours after starting the reaction, the total pressure is 2 MPa, the fluidization speed is always 52 cm / s and the height of the fluidized bed is 12 m. Furthermore, the reaction temperature is 75 ° C. and the content of titanium in the polymer

manufactured is 3 ppm.

  s At this time, the ethylene introduction rate is 0.5 T / h.

  Then, every hour, the catalyst flow rate is increased by 50 g / h. At the same time, the ethylene flow rate increases by 500 kg / h. At this stage, we do not proceed to the control of the rate of introduction of hexene-11; on the contrary, the aim is to maintain an identical gaseous composition by carrying out a control in

  maintaining constant the ratio of the partial pressures of comonomer to monomer.

  Under these conditions, we first obtain a copolymer whose density is significantly lower than the target value (0.916) as shown in the figure there are agglomerates of very low density (<0.912) which appear. We are then obliged to reduce the production to 10 t / hour and to modify the flow rate of hexene-1 manually so as to maintain the density at a more or less correct value while never reaching the degree again. density control obtained by the method of the present invention. Stops even had to be made in order to

clean the reactor.

-14-

Claims (9)

  1. Process for controlling the density of copolymer produced by a process of copolymerization of at least two alpha-olefins having from 2 to 12 carbon atoms in a gas phase polymerization reactor in the presence of a catalyst of metallocene type characterized in that control is obtained by maintaining the ratio of the flow rates of introduction of comonomer (s) to the constant monomer. 2. Process for controlling the density of copolymer produced by a process for the copolymerization of alpha-olefins having from 2 to 12 carbon atoms in a polymerization reactor in the presence of a polymerization catalyst characterized in that the ratio of the concentrations molars of the comonomers to the monomer in the copolymer produced is greater than the ratio of the partial pressures of the comonomers to the monomer and in that the control is obtained by keeping the ratio of the flow rates of introduction of comonomer (s) to the monomer constant.   3. Method according to any one of the preceding claims, in which the   alpha-olefins are chosen from ethylene, propylene, butene-1, hexene-1,   4-methyl-1-pentene and 1-octene-1.   4. The method of claim 3 wherein the monomer is ethylene or propylene, and the comonomer is propylene, butene-1, hexene-1, methyl-4 pentene-1 or octene-1.   5. Method according to claim 4 wherein the monomer is ethylene and the   comonomer is butene-1, hexene-1 or methyl-4 pentene-1.   6. The method of claim 5 wherein the comonomer is hexene-1.   -15- 7. The method of claim 2 wherein the reaction is carried out in a reactor   gas phase polymerization.   8. Method according to claims 1 or 7 wherein the polymerization reaction   in the gas phase is carried out in a fluidized bed reactor.   9. The method of claim 2 wherein the polymerization reaction is carried out in the presence of a catalytic system comprising a type catalyst metallocene.